Trunked communications systems are well known in the art. Similarly, networking of such systems for expanded coverage, hereinafter referred to as trunked communications networks, is known. Generally, trunked systems share limited communication resources, which are typically radio frequency (RF) channels, amongst a relatively large number of communication units, or subscribers. For purposes of this disclosure, a subscriber shall refer to any mobile radio, portable radio, console, or any other similar communication device known in the art. After registering onto a trunked system, a subscriber is able to communicate with any other subscriber via a selected voice, or data, channel which is selected by the repeater, or transmitter site. Communication between the transmitter site and subscriber is typically done using a selected RF channel, commonly known as a control channel. Control channel traffic is comprised of inbound signalling words (ISW) and outbound signalling words (OSW). A subscriber issues an ISW to request use, or relinquishment, of a voice channel, while the repeater uses an OSW to respond to such requests. Inbound traffic is comprised of subscriber ISW's as well as voice, or other data, in either digital or analog form, sent by a subscriber initiating a communication. Outbound traffic is comprised of repeater OSW's as well as data, as described hereinabove, to be delivered to a target subscriber. In a trunked communications network, a central processor, or hub, routes inbound and outbound traffic among trunked systems, for distribution to/from the respective subscribers within their coverage areas.
As part of the registration process, the registered subscriber is assigned an identification code (ID) by the system. Subsequent call requests by the registered subscriber include this ID as part of the inbound message, which is transmitted to the repeater. The inbound message is then forwarded to the hub, typically via a wireline, such as a packet data network, or PDN. When not participating in a call, each subscriber monitors the control channel. When the transmitter site receives a call request ISW from a source subscriber (e.g., to initiate a communication with a target subscriber, or subscribers), the site issues a channel grant OSW to the source and all target subscribers, which includes the frequency of the available voice channel, if such a channel is available. Upon receipt of the channel grant, both the initiating subscriber and the target subscribers leave the control channel frequency and move to the assigned voice channel frequency to participate in the call. If, however, all voice channels are in use, the site returns a busy OSW and the source subscriber must wait until a channel becomes available. Call requests, channel grants, and busies are only a select few of the many types of inbound and outbound traffic which characterizes a modern trunked communications system, and network of systems.
The RF range of a single-site trunked communications system is typically on the order of 20 miles from the transmitter (i.e., a system RF coverage diameter of 40 miles). Within this area, it is assumed that the mobile and portable subscribers are allowed to move about, or roam, at will. If a subscriber should roam outside the RF coverage area of the trunked system, the subscriber would be unable to acquire the control channel from the transmitter site, for that system, rendering communication with (to/from) that subscriber impossible. Accordingly, the subscriber goes into a scan mode to acquire a control channel from another site. Therefore, in a multiple-site trunked communications network, such as the one shown in FIG. 1A and later described, the subscribers are capable of roaming between a plurality of RF coverage areas in the network, substantially without loss of communication capability.
At least one well known method exists to provide trunked communications in a networked configuration. In such a network, subscribers have nationwide roaming capability through use of selected trunked systems which are equipped with local computers that communicate with a national hub computer. At each selected trunked system, several IDs are reserved as "roaming IDs" to be temporarily assigned to roaming subscribers. When a subscriber determines that it has roamed into a new trunked system, it requests a roaming ID from the local host system. The roaming ID assignment is transmitted to the hub computer so that appropriate calls may be forwarded to/from the roaming subscriber. The roaming subscriber can operate under the assigned roaming ID until it roams out of the range of the current system and into yet another trunked system. In this way, the subscriber is allowed to roam from system to system.
FIG. 1A shows a graphical representation of a simple three-site trunked communications network. Such a network may be comprised of three coverage areas 104-106, which are served by hosts, or transmitter sites 107-109, respectively. These transmitter sites are coupled to a central processor (CP) 110, via wirelines 114 for example. CP 110 has coupled to it a memory device 112 which is typically used for storing network and subscriber information, which is later described. When a subscriber, for example subscriber 102, desires to initiate or receive calls through the trunked communications network, a roaming ID assignment is required for that subscriber. Such an ID allows the hub to keep track of that subscriber and determine an appropriate transmitter site to which outbound messages should be delivered.
FIG. 1B shows a simplified ID-assignment table for the subscriber 102 shown in FIG. 1A, which table is known in the art. Such a table will be used to illustrate how the ID is tracked by the host sites as the subscriber roams across their respective coverage boundaries. At a time T1, the subscriber enters coverage area (CA) 104, registers with transmitter site 107, and is assigned an identification code (ID) of 5, for example. After traveling through CA 104, at a time T2, subscriber 102 de-registers with transmitter site 107, and relinquishes his roaming ID so that it can be re-issued to another subscribers at a later time. Upon entering into CA 105, and explicitly registering with site 108, subscriber 102 is assigned ID 3, for example. This temporary, roaming ID is used until a time T3, when subscriber 102' enters into CA 106. At that time, the de-registration-registration sequence occurs again, whereby ID 3 is relinquished to the system, and a new ID 6, for example, is issued by site 109 to subscriber 102' for purposes of communicating within CA 106. At a later point in time, subscriber 102' may turn around and re-enter CA 105. By doing so, the unit must, at a time T4, de-register with site 109 and re-register with site 108. Though subscriber 102 has previously been in coverage area 105, its previous ID (e.g., 3) is no longer available for its use. Accordingly, a new ID assignment is necessitated and site 108 issues ID number 9 to subscriber 102" for purposes of communicating within coverage area 105.
The aforementioned scenario, though exemplified in a simple, three-site system, serves to show the tremendous amount of control channel traffic required by a system which re-assigns ID numbers, as shown in the known systems of the prior art. The time-consuming exercise of first de-registering (unless not yet registered) and subsequently re-registering upon crossing coverage area boundaries, represents an undesirable ratio of control channel traffic to that of actual communication (e.g., voice, data) between subscribers. Furthermore, in the case where a subscriber rarely initiates or receives messages, the task of repeatedly de-registering and re-registering while roaming throughout the system becomes a frivolous, unnecessary exercise. From the perspective of the service provider, control channel traffic simply amounts to non-billable, administrative overhead.
Accordingly, there exists a need for a trunked communications network which does not rely on explicit registration and de-registration to keep track of roaming subscribers within that network. Further, it is desired to have a system which uses subscriber-specific parameters to more efficiently allocate communication resources to those subscribers.